Restore traditional single-pane windows

Final finishing

Apply the layer of putty that seals out the weather. Use the same smearing technique, but this time, hold the putty in one hand and use a putty knife to squish it into the rabbet. You should have a sausage-shaped lump of putty going. Hold the end of the sausage up to the rabbet and use the tip of the knife to push a chunk into it. It doesn’t have to be neat looking. What you’re aiming for is good contact between the putty and the sash and glass. Don’t be afraid to use lots of putty-you’ll clean up the extra. Work along until you have the rabbet filled. Then, draw the knife along with one hand with the knife tilted at an angle, from the outside edge of the rabbet to the inside edge (on the other side of the glass), and apply pressure to the tip of the blade with the other hand. Excess putty will be pushed over the outside edge of the rabbet and onto the glass, leaving a nicely formed fillet-or at least it will after a few practice runs. The tricky part is making a nice, compound bevelled corner where the two lines of putty meet. There’s no magic though. You simply give your knife a twist and shape the soft putty into the correct shape. It’s forgiving stuff. You can go over it again to smooth it out if you’ve made an error.

Let the putty dry thoroughly before painting, waiting until it has cured hard, and ensure that the paint overlaps the putty bevel and goes onto the glass by 1„16″ to seal the putty.